Polyurethanes have been produced by reaction of a polymeric polyol with an organic diisocyanate and as required a low molecular compound having more than two active hydrogen atoms. As the polymeric polyols being used are the ones of ester type such as polyester polyols prepared by polycondensation of a dicarboxylic acid component such as aliphatic dicarboxylic acids including succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, etc., with a glycol component such as ethylene glycol, propylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, etc., and polycarbonate polyols prepared by ester exchange reaction of 1,6-hexanediol with diphenyl carbonate. ( Keiji Iwata: POLYURETHANE RESINS, p.56 to 61, Nikkan Kogyo Shinbunsha, published on July 30, 1975)
However, the polyurethanes prepared from the above polyester polyols are inferior in resistance to hydrolysis, and thereby surfaces o the films formed from the polyurethanes will become sticky or will crack after a shoft period of time. Therefore, this type of polyurethane is limited in its application. In order to improve resistance to hydrolysis of such polyurethanes it is effective to make lower the concentration of ester group in the polyester polyol residue existing as the polymeric polyol residue in the polyurethane. For this purpose it is preferred to use as the polymeric polyol a polyester polyol prepared from a glycol and a dicarboxylic acid both having a greater number of carbon atoms. However the polyurethane thus obtained from such polyester polyol has, though excellent in resistance to hydrolysis, a greater tendency toward crystallization, and when left standing for some time under an atomosphere of low temperature lowers flexibility as represented by bending resistance.
On the other hand, polyurethanes prepared from the afore-mentioned polycarbonates as the polymeric polyol are, though excellent in resistance to hydrolysis, inferior in low temperature characteristics because they have high coagulation temperatures, and also because they have high glass transition points and at the same time have a great tendency toward crystallization, as is typical in the case of most widely used polycarbonate obtained from 1,6-hexanediol.
In order to solve these problems and to obtain a polyurethane being excellent in both resistance to hydrolysis and low temperature characteristics, the inventors have proposed in EPA-194452 Specification or JPA 195117/1985 a method comprising using 1,9-nonanediol to improve resistance to hydrolysis as the glycol for preparing polyester polyol or polycarbonate polyol to be reacted with a polyisocyanate, and using in combination therewith 3-methyl-1,5-pentanediol to prevent a decrease in low temperature characteristics caused by an increase in tendency toward crystallization attendant upon the use of 1,9-nonanediol. Though this method is an excellent one, it is often the case that, depending on the end-use, a still higher degree of satisfying both resistance to hydrolysis and low temperature characteristics is required. Particularly when used for artificial leathers, synthetic leathers, etc., it is required to satisfy the above requirements while using a soft polyurethane, and moreover it is strictly required to give the polyurethane such characteristics at low temperature as surface properties, particularly bending resistance, resistance to surface wear, etc. In such cases the improvement effect proposed by the present inventors in the above-cited method will not be sufficient. For instance a polyurethane having soft constitution, that is, one in which the content of hard segments is small or in which the molecular weight of soft segments are high, crystallization degree of polyester polyol or polycarbonate polyol will increase, and a small amount of addition of 3-methyl-1,5-pentanediol can not fully suppress the tendency toward crystallization of polyurethane attributable to 1,9-nonanediol, resulting in an inferiority in low temperature characteristics. There is another method of increasing the amount of the co-used 3-methyl-1,5-pentanediol in the preparation of the raw material polyester polyol or polycarbonate polyol to give non-crystallisability. In this case resistance to hydrolysis of the obtained polyurethane decreases, and besides, the glass transition temperature of the raw material polyester polyol or polycarbonate polyol itself becomes higher, resulting in creation of a problem of decrease in low temperature characteristics of the obtained polyurethane.